Source : Webster's Revised Unabridged Dictionary (1913)

Alertness \A*lert"ness\, n.
   The quality of being alert or on the alert; briskness;
   nimbleness; activity.

Source : WordNet®

alertness
     n 1: the process of paying close and continuous attention [syn: {watchfulness},
           {vigilance}]
     2: a state of readiness to respond; "alerting was indicated by
        the desynchronization of the EEG" [syn: {alerting}]
     3: lively attentiveness [syn: {on the qui vive}]